LICHTMAN: Earlier this week, NASA announced plans to launch another rover to Mars in the year 2020. And there's some buzz, there's some speculation that this one could have a wheel up on Curiosity. Maybe it wouldn't just analyze samples there but could shift them back to Earth.

FLATOW: You know, I've heard that before.

LICHTMAN: Don't be downer, Ira.

(LAUGHTER)

FLATOW: I am not going to get - in fact, we're waiting for that to happen. It was supposed to happen. But on - I'm going to look on the positive side because the objectives of this new formation had not been decided to yet. And, you know, the entire project is still going to depend on NASA's funding in Congress.

LICHTMAN: OK, OK.

(LAUGHTER)

FLATOW: We know what funding in Congress is like. So whatever let's ask our audience, right?

LICHTMAN: Yeah.

FLATOW: Let's ask them. What do you think? What exciting things - if they're building the Curiosity twin now, they haven't launched it yet, what do you want see on that baby? What do you want it to do maybe that the other, the Curiosity, is not doing? Or maybe we're too focused on Mars, you want to go some place else. What do you think?

LICHTMAN: 1-800-989-8255. 1-800-989-TALK. And let's introduce our guest who may actually be able to speak to this with authority. Jim Green is the director of NASA's Planetary Science Division. And he's joining us from a studio at NASA headquarters in D.C. Welcome to the show.

JIM GREEN: Well, thank you very much. It's a pleasure to participate in this great program.

LICHTMAN: What are the plans for this rover? So far, what do we do know? What should we expect?

FLATOW: What will you tell us?

(LAUGHTER)

LICHTMAN: OK.

GREEN: You know, I'll tell you anything I can about this rover, because we're really excited about the science rover that we have planned (unintelligible).

GREEN: But we have plenty of time to name it and move along. But this rover will have a major science objective. You know, we've been over the years developing a program where we look at Mars to find water, you know, sort of follow the water. It's one of those - this essential ingredients about life. And now, we know. Just about everywhere we look there are signs of past water and, in fact, we've even - we believe observed water emanating from aquifers down crater walls, that's been a really exciting discovery over just last year.

So from that perspective we're moving on. Following the water, the answer is we found it. And now, we're going to move into an era of seeking signs of life. You know, beyond Earth, we want to know if there are modern habitats elsewhere in the solar system where there are necessary conditions for life. That means organic matter, water, energy, nutrients - those things that allow life to develop and be sustained. And Mars actually used that - is that gem of a planet that's close by, that enable us actually to interrogate to find the answer to that question. And that's what (unintelligible).

FLATOW: And how will you find the answer to that? You're going to bring any of it back or send it back like the defunct Mars return mission that never happened? Or are you going to analyze it in greater depth? Or what are you going to do?

GREEN: OK. We're going to take a chapter out of what we've learned from the Apollo missions. And that is, indeed, be able to identify, and grab samples and bring those back. In fact, we're still analyzing lunar samples. We recognized that samples back from places like the moon and, of course, Mars provide us a scientific treasure throve of information that will only improve as instrumentation and our ability to interrogate it improves over time. And so...

FLATOW: Do you have a budget set this for this yet? Is there any money?

GREEN: Yes, we do. In fact, what we announced this week at the American Geophysical Union is what we would call an in guideline from the president's budget submission that he gave Congress in February of this year, a rover in 2020. Now, the ability to get it at a cost that we can afford is really predicated on tremendous effort that's going on with the successful landing of Curiosity. You know, being able to get one ton down on the surface in such unbelievable and fantastic way.

We're going to be able to reproduce most of that infrastructure changing up the experiments to where we really ago after the life issue, determine if life ever arose on Mars. Go after more of the potential organics that are there and to be able to cache samples, potentially return those in the future but certainly interrogate those, create those from coring into rocks or drilling under the soil, and get to a point where we're actually looking in the past of Mars to determine how that environment has really changed over time.

FLATOW: Jim Green, I want to thank you for taking time to be with us today. One more question for you, from Flora.

LICHTMAN: No. Stay on because we have - we want you to talk with our next guests too. So we've been talking about Mars, but there's also this really exciting news from Mercury this week. Last week in the journal Science, scientists reported that NASA's Messenger spacecraft found evidence of water ice on Mercury. Remember, Mercury is a scorcher, right? It's daytime temperature is 800 degrees. It's the closest...

GREEN: Yup.

LICHTMAN: ...planet to the sun. And yet it also harbors billions of tons of frozen water, these studies say.

FLATOW: Wow.

LICHTMAN: And our next guest is here to talk about this discovery. Matthew Siegler is a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, and he's the co-author in one of the three papers about Mercury that appeared in the journal Science. Welcome to the show.

MATTHEW SIEGLER: Hi. How are you doing? I'm actually at the AGU conference that he was talking about earlier. We've heard, also, the neat, exciting stuff about planets this week.

FLATOW: Now, how was it possible that there can be so much water on this giant - on this teeny, hot planet?

SIEGLER: Oh, well, it's hot when it's under the sun. It's pretty cold when it's out of the sun and because Mercury has essentially no tilt, places near the poles can stay in permanent shadow and be hundreds of degrees below zero. So we're measuring things down to about 50 Kelvin or modeling things down to about 50 Kelvin. We don't actually measure any temperatures.

FLATOW: And there's a lot of water. Was I reading something about if you put it on the island of Manhattan it would be 10 - how many miles deep and spread out?

SIEGLER: Yeah, yeah. The one I liked was if you covered the Washington, D.C. area, it would be about two miles thick, probably, of ice. So it's pretty impressive.

FLATOW: Can we speed that up a little bit, covering that D.C. area?

LICHTMAN: Well, what about, you know, we've been talking about follow the water, follow the water. But here, seems like there's a lot of water. Why aren't we following this? You know, what's the case for Mars, Jim Green, over Mercury?

GREEN: Well, indeed Mercury is tough to get to. You know, it took us about six years to get Messenger launched and lining up and getting in orbit so that it could do the fantastic measurements that it's made, whereas Mars is much more accessible. We line up every 26 months so our trajectory can get to Mars quite easily in about a nine-month period.

FLATOW: This is SCIENCE FRIDAY, from NPR, talking with Jim Green and Matthew Siegler. Matthew, is there a - could there be a sweet spot in that shadow that gets really close to the sun but, you know...

SIEGLER: Oh, this is what everyone is asking about.

(LAUGHTER)

SIEGLER: So basically, you got to think about what happens to a water molecule on a body with no air, all right? So here it is. Maybe - we're - this ice we're measuring, we can only see in the top meter, so maybe it goes down 10 meters. We're really not sure yet. That would take future measurements. But if you're that close to the surface, if you get even a little bit warmer than about something like 100 Kelvin - so that's real cold, right - you're going to sublimate off into space in less than an Earth day. So these are pretty short time scales to survive around.

FLATOW: Mm-hmm. So no hope of finding any liquid water there.

LICHTMAN: But there was organic matter there, too, right?

SIEGLER: Yeah. So that's actually the really exciting thing from this. So the ice - we've, sort of, known that it's been there since about 1992 when we did radar measurements from (unintelligible). There has been this evidence that there was this flight reflective material down there. So the question was, was it really water ice? And the Messenger mission has been able to prove that. But it also had this laser altimeter that's been a surprise finding to it. It's just meant to measure topography

But it's all - it shot a laser at some of these areas where we predict that, thermally, there should be ice, stable at the surface. It's all bright stuff. But what was really neat was where ice should be under the surface, we saw black stuff. And when we looked at the temperatures that this should be stable under, there are a bunch of organic materials that we see on the surfaces of comets all over the solar system that fit this description perfectly, and not many other materials would do. So it's pretty cool.

FLATOW: So it could have been bombarded by comets and brought the water and the dark stuff there?

SIEGLER: Yeah, that's sort of neat to look at the origin of the water on Mercury. And we're predicting that it's most likely from comets, because we see a other material with it. And then that tells us a lot about how water came to the Earth, right? We think that the whole, you know, solar system should have been dry when the solar system formed out of a big cloud of gas and dust, right?

And so somehow the Earth has water and Mercury has water. The moon seems to have a little water. Where did this come from? And we're thinking in this in this paper series that comets might be a good source of that.

LICHTMAN: Is there any chance you could home grow water? I mean, does it have to be shipped in? Could you make it on the planet?

FLATOW: Could it outgas from the center of the planet somewhere or volcanoes or some...

SIEGLER: Yeah. And so the Earth, right, has more water inside of it than it does on the surface, so yeah, that could be a source. And so maybe it could tell us something about the internal structure of Mercury and then it had water vapor in there. But the fact that we see this other stuff with the water is strong evidence that this is water coming from somewhere else. We really have to land there and sample it and maybe bring it back, like Jim was talking about with Mars, to really answer some of these questions.

FLATOW: I see you writing the proposal as we speak.

SIEGLER: If I don't, someone will, right?

(LAUGHTER)

FLATOW: All right. Thank you, gentlemen, both of you. And good luck because we're going to wait for that mission - the return mission to Mars with a new post - maybe we can name it on SCIENCE FRIDAY. What do you think, let us help you out, Jim, do that?

GREEN: Oh, you've got to have me back and we'll discuss more of that and maybe we can do something.

FLATOW: All right.

LICHTMAN: We'll look forward to it.

FLATOW: And good luck to you, Matthew, in getting that mission to Mercury to go down and sample that.

SIEGLER: All right. Well, thanks for your interest in this forgotten planet.

FLATOW: Not anymore. You made the head of the news this week, so I think Mercury is in the news.

SIEGLER: Right. So that keeps it from being knocked off like Pluto, right?